A fiber-reinforced composite material comprising matrix resin and reinforcing fibers has been increasingly used to produce various shaped products in various fields year by year because of its high mechanical properties.
A composite material comprising carbon fibers specifically having excellent performance among reinforcing fibers can generally be formed by an autoclave molding method in which semi-hardened intermediate base materials called “prepreg” made by impregnating continuous carbon fibers with matrix resin are laminated to be heated and pressurized with high-temperature and high-pressure pot to harden the matrix resin of a continuous fiber-reinforced composite material. Recently, for the purpose of improvement of the production efficiency, RTM (Resin Transfer Molding) method in which continuous fiber base material preformed in a shape of member in advance is impregnated with matrix resin to be cured has been employed. The carbon fiber composite materials provided by those molding methods have excellent mechanical properties derived from continuous fibers. The continuous fiber having a regular layout and a small variation of mechanical properties can be designed to have necessary mechanical properties by placement of the base material. However, a complicated shape having a three-dimensional shape or the like cannot easily be formed with continuous fibers while a member in a flat shape can easily be formed.
Complicated shapes such as three-dimensional shapes can be formed with an SMC (Sheet Molding Compound) or a stampable sheet. SMC shaped product may be prepared by heating and pressurizing a semi-hardened sheet-like base material (SMC) by using a heating type pressing machine, wherein the sheet-like base material is made by impregnating the chopped strands made by cutting carbon fiber strands into 25 mm of fiber length with thermosetting matrix resin. The stampable sheet shaped product may be prepared by cooling and pressurizing a sheet-like base material (stampable sheet) in a mold at a predetermined temperature after heating the sheet-like base material by using an infrared heater or the like above the melting point of the thermoplastic resin, wherein the sheet-like base material is made by impregnating chopped strands cut into about 25 mm or a nonwoven fabric mat or the like with thermoplastic resin.
In many cases, SMC and stampable sheet cut into a size smaller than that of the product shape before pressurization are placed on a mold and drawn out (as being flowed) in the form of shaped product by pressurization to perform molding. Therefore it is able to follow a complicated shape such as three-dimensional shapes by the flow. However, the nonwoven fabric mat or chopped strand might have uneven distribution and uneven orientation in the process of making a sheet of SMC or stampable sheet to decrease or vary the mechanical property. The uneven distribution and uneven orientation might cause a sink mark, warping or the like, specifically for a thin member.
To reduce the defects of the material described above, for example, WO 08/149615 discloses chopped fiber bundle and its molding material having a section in which the number of reinforcing fibers increases in the orientation direction of the reinforcing fibers toward the center of the chopped fiber bundle from starting points at both ends of the reinforcing fibers in the orientation direction, although sufficient study has not been accomplished about arbitrarily changing the cutting angle according to length of reinforcing fiber bundle.
Documents
Patent Documents
Patent document 1: WO 08/149615
Accordingly, it could be helpful to provide a fiber-reinforced composite material that can achieve both high fluidity and high mechanical characteristics during molding at a level higher than that could be achieved with a conventional fiber-reinforced composite material comprising reinforcing fiber and matrix resin so that excellent uniformity of fiber-reinforced composite material, excellent flow-molding fluidity and excellent mechanical characteristics are achieved.